pull in base/stringprintf.h explicitly

Newer libbase no longer implicitly includes the stringprintf.h header,
so building against it fails with missing StringPrintf definitions.

BUG=chromium-os:25872
TEST=`emerge-x86-alex update_engine` works with old & new libbase

Change-Id: Ia1ea842449c0382ee4c8e5abd3ddfb3012c3ff2f
Reviewed-on: https://gerrit.chromium.org/gerrit/19774
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/prefs_unittest.cc b/prefs_unittest.cc
index 87116e5..54a637e 100644
--- a/prefs_unittest.cc
+++ b/prefs_unittest.cc
@@ -1,4 +1,4 @@
-// Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
+// Copyright (c) 2012 The Chromium OS Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
@@ -9,6 +9,7 @@
 #include "base/basictypes.h"
 #include "base/file_util.h"
 #include "base/string_util.h"
+#include "base/stringprintf.h"
 #include "gtest/gtest.h"
 #include "update_engine/prefs.h"